Self-starting induction motor



Dec. 4, 1928. 1

K. L. HANSEN ET AL SELF STARTING INDUCTION MOTOR File d Aug. 15, 1926 g mwq 4% 1???? m as a; 2 afifw Patented Dec. 4, 1928.

UNITED STATES PATENT OFFICE.

KLAUS L. HANSEN AND WILLIAM J. OESTERLEIN, OF MILWAUKEE, WISGONSIN.

SELF-STARTING INDUCTION MOTOR.

Application filed August 13, 1926. Serial No. 128,948.

This invention relates to induction motors. In general, this invention is an improvement over that disclosed in our co-pending application for self-starting induction motors, filed,

I July 10, 1926, Serial No. 121,603, and has the same objects as those set forth in such applica- In induction motor practice, it is well known that high starting torque may be secured by introducing resistance in the secondary circuit, and attempts have been made,

- as is evidenced by. the double squirrel cage motor, to provide two distinct squirrel cages, one of which has its conductors located adj ai cent the periphery of the rotor, and the other of which has its conductors embedded deeply within the rotor, the first cage being of relatively high resistance, and the second of relatively low resistance.

) It is obvious that this construction necessitatesspecial dies and does not provide for any adaptability of the motor to diiferent requirements, as a complete redesign would be necessitated in any change in its characteristics.

3 These motors also have the disadvantage that for high current values in the rotors the leakage fiux paths surrounding the low resistance conductors become quickly saturated and, consequently, do not prevent the inrush of 3 current as it should for starting.

However, it has already been stated that when the magneticpath of the leakage flux of the inner cage is a closed magnetic circuit, the path becomes saturated and the induci tance decreases for large current values. To prevent this it becomes necessary to introduce an air gap in the path of the leakage flux. This may be accomplished by slotting the iron between the outer and inner slots.

1 But'the introduction of an air gap in the path of the leakage flux increases the reluctance of the path, and to keep the reluctance down to a low value, the area of the cross-section of the path must be increased. That is,

5 for the same length of core of the rotor, the radial distance must be increased, which,in turn, increases the rotor diameter. Besides these disadvantages, special dies have to be employed which necessarily materially increase the cost of construction.

This invention is designed to overcome the defects noted above,'and objects of such invention are to provide a self-starting induction motor in which the rotor is so constructed that a single squirrel cage is employed, and

yet the starting current is held to a relatively low value as compared with the usual practice, and in which the running characteristics of the motor are vastly improved over anything ieretofore known in self-starting motors.

Further objects are to provide a novel form of squirrel cage induction motor in which the motor may be most easily modifiedeven after its majorportion has been constructed so that it will fit a large variety of requirements without requiring any redesigning of its main parts.

Further objects are to provide an induction motor having a single squirrel cage in which two paths are provided for the current, one of which is of high resistance-and the other of low resistance, and in which means are provided for forcing the starting current to follow the high resistance path and allowing the current during running conditions to follow the low resistance path.

It is to be noted that the reactance is a qfuantity containing both the frequency and t e inductance as factors. It is, therefore, a further object of this invention to prevent a rise in inductance as the motor speeds up or, what is the same thing, to prevent the inductance from decreasing as the motor slows down.

With constant inductance the reactance, therefore, decreases directly with the frequency. The current is, in general, higher at standstill than when the motor is running,

and it is, therefore, essential to keep the inductance from increasing as the motor speeds up, or what is the same thing, to keep it from decreasing as the motor slows down and the current increases.

Further objects are to provide an induction motor in which a squirrel cage rotor is employed and is so constructed that the current flowing in the squirrel cage is controlled by simple means, easily accessible and ad ustable to the exact demands to which the machine will be subjected, and in which the running characteristics of the motor are materially improved, as well as the starting characteristics.

Further objects are to provide a novel form of construction in which the means employed for governing the reactance of the secondary circuit is located wholly at the end or ends of the rotor and is thus very readily changed to give a wide variety of characteristics to the motor.

Embodiments of the invention are shown in the accompanying drawings in which:

Figure 1 is a diagrammatic view in section showing the rotor of an induction motor constructed in accordance with this invention;

Figure 2 is a similar view showing a modified form;

Figure 3 is a further modified form in which the low resistance end ring employed in the other forms has been omitted;

Figure 4 is a still further modified form.

Referring to the drawings, particularly Figure 1, it will be seen that the rotor is indicated by the reference character 1 and the stator by the reference character 2, the rotor being carried by the usual rotor shaft 3. This rotor is provided with slots or holes adjacent its periphery within which the conductors 4 of the squirrel cage are positioned. The conductors at one end are joined by'a low resistance end ring 5. At the other end the conductors are joined by a low resistance disc-like end ring 6, and intermediate the end of the rotor and such last mentioned end ring a high resistance end ring 7 is provided. The end ring 6 is held between two sets of laminated discs 8 and 9 of magnetic material which cover the radial face of the end ring 6.

In actual operation, it has been found that the current at starting due to the high frequency induced in the rotor conductors 4 does not pass through the end ring 6, but instead, passes through the high resistance end ring 7. The current is kept from passing through the end ring 6 by the fluxes carried by the discs 8 and 9. Obviously, the exact proportioning of the current in the two end rings 7 and 6 may be secured by varying the radial depth of the discs 8 and 9. For example, one or both ma stop short of the shaft 3 and either set 0 discs may extend more or less beyond the ends of the end ring 6. Further, the cross sectional areas may be varied to thus exactly adjust the inductance to the exact needs of the machine.

It is apparent that the flux will travel radially through the discs 8 and 9 and jump the two end gaps at the inner and outer peripheral portions of such end ring and across the relatively extensive opposed faces of the discs. The proportiom'ng of the air gaps determines the reluctance of this auxiliary flux path. The air gaps obviously can be controlled in a number of different ways. For example, they may be controlled by the thickness of the end ring which thus determines the spacing of the discs 8 and 9. Further, the air gaps may be also controlled by varying the area of the extended portions of the discs 8 and 9.

Thus a wide control of the characteristics of the motor is afforded by this invention.

It is, however, to be particularly noted that the construction is extremely simple and may be easily followed in ordinary machine shop practice without requiring special dies or special redesigns of the motor. Instead, the main portions of the motor remain the same, although the characteristics of the different motors may be mgst easily controlled.

It is apparent that the construction outlined in reference to Figure 1 and shown at one side of the rotor only, can be duplicated at opposite sides of the rotor, as shown in Figure 2. In this case, two relatively high resistance end rings 7 are employed and two low resistance disc-like or ring-like end rings 6 are used. The end rings 6 are in each case positioned between the magnetic discs 8 and 9.

Under certain conditions where high starting torque is not required, the construction may be further simplified, as illustrated in Figure 3. In this form of the invention the high resistance end rings are dispensed with, and the conductors 10 are directly joined at their ends to the disc-like end rings 11. These end rings are positioned adjacent the ends of the rotor body 12 and magnetic discs, preferably of laminated material as indicated at 13, are positioned outside of these low re sistance end rings.

In this form of the invention, the auxiliary magnetic circuit traverses a portion of the rotor body itself, and also the end discs.

This construction is simpler than that previously described, and produces the requisite inductance for the secondary circuit for the high frequency starting currents. However, the low frequency running currents are not hampered, as the reactance falls off as the speed of the motor increases.

This distortion of current in the end ring is, of course, most pronounced at high frequency and becomes negligible at low freqpenc The effect is that at hi h frequency t e e ective resistance of the en ring is considerably higher than at low frequency. This effect is so pronounced that in some a plications the high resistance end rin can dispensed with, sufiicient variation in resistance from standstill to normal speed being obtained by distortion of the current in the ring 11 itself.

This construction, therefore, may be resorted to where characteristics of the above outlined t pe are desired.

In the fi irm shown in Figure 4, a portion of the rotor body 14 is indicated as provided with the usual squirrel cage conductors 15, similar to those previously described. These conductors pass to end rings 16 of low resistance which are positioned between ring-like magnetic discs or members 17 and 18. The magnetic path may be completed by means of either outer magnetic rings 19 or inner ma gnetic rings 20, or both these rings may be used.

In this construction, it is to be noted that the inductance is very much increased for the end rings, as a substantially closed mag scribed above, without altering the essential construction of the motor.

It is to be particularly noted that no special dies are required in the construction of these motors. The ordinary discs forming the laminations of the rotor body may be employed, if desired, or ordinary magnetic rings may be used.

Although the invention has been described in considerable detail, such description is intended as illustrative rather than limiting as the invention may be variously embodied and as the scope of such invention is to be determined as claimed.

We claim:

1. An induction motor comprising a stator and a rotor, said rotor having a squirrel cage with the conductors joined by end rings, one of the end rings having a large radial depth,

said rotor having magnetic material positioned on opposite sides of said last mentioned end ring andhaving opposed faces lying in planes at right angles to the rotor axis and spaced apart throughout their extent, said magnetic material being relatively stationary and extending at least to the outer edge of the last mentioned ring, the magnetic flux due to current in the last mentioned end ring passing between the said opposed faces and through such end ring, whereby the current path in the last mentioned end ring is distorted and the effective resistance of such end ring is increased at starting. I

2. An induction motor comprising a stator and a rotor, said rotor having a squirrel cage with the conductors joined at their ends by flat end rings having a' relatively large radial depth, said end rings lying closely adjacent the end faces of the rotor, magnetic discs located at the outer sides of the end rings and extending at least to the outer edges of said end rings, said rotor and said discs having opposed, spaced, fiat faces, the magnetic flux due to the current in the end rings producing a flux passing between the opposed faces and through the end rings, whereby the current path in the end rings is distorted and the efl'ective area of the end rings is decreased and the efi'ective resistance of the end rings is increased.

In testimony that we claim the foregoing we have hereunto set our hands at Milwaukee, in the county of Milwaukee and State of Wisconsin.

' KLAUS L. HANSEN.

WILLIAM J. OESTERLEIN. 

